The number of telephone addresses available to subscribers in the United States is limited to the combination of allowable digits in the address array. For larger cities the earliest combination was simply four numbers n EQU nnnn (1)
for each telephone address, allowing almost 10,000 addresses, which served well enough for a limited market with a single exchange. With telephones proliferating however additional exchanges were required, particularly with the advent of automatic dialing. With the interconnection of exchanges not only within the service area of a particular telephone company but between telephone companies it was soon obvious to telecommunication engineers that a telephone address convention would have to be adopted incorporating uniform exchange identifiers into telephone addresses.
According to the convention agreed upon, exchanges would be labeled by names using a three letter alphabetical identifier. Alphabetical letters would accompany each number on the dial as Alphabetical letters would accompany each number on the dial as shown in FIG. 1A. The exchange names would comprise three letters eee that would serve as a mnemonic: by separating the telephone address into the two groups of alphanumeric characters subscribers could more easily remember their telephone addresses. The exchange identifier would precede the four-digit telephone address. This expanded seven-digit sequence is generally represented by two bound arrays of characters with a divider: EQU eee-nnnn (2)
The number of available telephone addresses increased, but not too significantly because each three-letter group still had the same numerical counterpart. To further increase the numbers of available addresses when expansion became necessary the final letter of each exchange identifier was subsequently changed to a number, increasing the available telephone addresses nine-fold for each exchange. EQU een-nnnn (3)
Automatic dialing was accomplished by actuating an electromechanical rotary switch in each telephone which sent a sequential number of audible pulses equal to each number-digit dialed to a ganged series of rotary switches actuated by stepping motors located at the local exchange.
The next step was direct long-distance dialing. The country was sectioned into discrete regions, each with a three digit area code ccc. Although dialing within any region did not require the area code, the complete telephone address for each subscribed now took the sequential form EQU (ccc) een-nnnn (4)
Fortunately the 1 and 0 numerals were not accompanied by letters and therefore no exchanges began with these dial positions. Local addresses do not begin with a 0 or a 1. Accordingly these digits were available to be used as an access code a. If the first digit dialed was either a 1 or an 0 this signaled the exchange that ten digits would follow, rather than the expected seven digits for a local call. EQU a (ccc) een-nnnn (5)
Telecommunications engineers had recognized that electromechanical switching was far too slow to accommodate the ever-increasing number of telephones in service, and moreover rotary switches were too prone to mechanical failure. Electronic switching was the logical next step. This was accomplished by assigning an audible dialtone to each dial number, with each telephone equipped with a dialtone generator. Consequently the dial itself could be replaced with key switches, as shown by the keypad in FIG. 1B.
Because stray tones on the telephone lines could trigger random dialing errors, two dialtone frequencies transmitted simultaneously: denoted multifrequencies, were specified for each switch arranged in a simple 4.times.4 matrix. A 4.times.4 matrix would require eight discrete frequencies, and the frequencies adopted, shown in hertz units in FIG. 1B, were well separated to eliminate misrecognition errors. The idea of discrete exchanges was dropped and each three-digit exchange address became simply one of almost 1000 numerical combinations.
This matrix arrangement accommodates 16 audible multifrequency combinations. This new arrangement allows an additional six switches to be added to telephone keypads when future demands required. These were designated the *, #, A, B, C and D keys. These additional keys will be referred to as symbol keys to distinguish them from the ten customary keys. Designating four of these symbol keys by letters however was a very poor choice indeed considering that these letters already appeared on the customary keys.
Although the letters Q and Z are still available other possible symbols such as @, &, % and $ have specific means that would again lead to perplexity. The expected confusion mitigated against adopting symbol keys in telephone addresses. As is often the case in progressive developments old conventions can hobble new adaptations. Only the * and # keys now appear on telephone keypads.
The only consequential advantage of electronic switching over electromechanical switching beyond speed is the availability of the multifrequencies to control automated devices attached to telephone lines, leaving the problem of the diminishing availability of addresses unsolved. With the proliferation of facsimile machines this problem has become acute. In virtually all commercial establishments an additional facsimile line with its own telephone address is now required in normal business practice, but the number of addresses available is still restricted by the sequence shown in Expression (3) for local usage and Expression (5) for long-distance usage.
The simplest solution to this problem would be to sub-divide the country into regions each with one of the almost 800 area code combinations allowable, which appears to be the direction the telephone companies are taking. This will however be quite confusing as far as distinguishing between local and long distance calls is concerned, and resented by subscribers because ten digits will be required for most calls. Of course the area codes can be eliminated altogether and each subscriber gets immediately a ten-digit telephone address instead of progressively using the first scheme. By eliminating the need for access codes moreover this would increase the available addresses by almost 1000. Again there would be strong public resentment against the ten digits required for all calls.
Another approach would be to lift the restrictions posed by Expressions (3) and (5) and simply add additional digits to telephone addresses as required. However this approach would require additional access codes to signal the number of digits to follow. Moreover the combinations shown in Expressions (3) and (5) are mnemonics, aiding in the memorization of a telephone address by establishing a fixed grouping arrangement of digits. Ultimately this latter approach would lead to an increase in misdialing as the number of digits required to reach any party will change with the address. It is as impractical as using the A, B, C and D symbol keys in telephone addresses.